1. Field of the Invention
The present invention relates to a magnetic recording medium in the form of tape. More particularly, the invention relates to a magnetic recording medium in the form of tape such as an audio cassette tape or a video cassette tape which is improved in running property, durability and electromagnetic conversion characteristics in the repeated use.
2. Description of Prior Arts
A magnetic recording medium in the form of tape (e.g., an audio-tape or a video-tape encased in a cassette) has been conventionally employed or stored in an environment having less temperature variation such as in an interior of house. However, as a radio cassette tape-recorder (i.e., a cassette tape-recorder with radio), a stereo record player for automobiles or a small-sized video tape recorder, etc. are widely employed in recent years, the magnetic recording media are now frequently employed or stored in a severe environment such as an environment of high temperature, environment of high temperature and high humidity, or environment of low temperature. For instance, it is reported that the highest temperature in an automobile interior is 104.degree. C. during day time in the summer season according to the high-temperature or low-temperature exposure test of automobile parts (JIS D 0204).
A magnetic recording medium exposed to such a high temperature is apt to shrink to curl, or to shrink more greatly on the edge of one side than that of the other side to have a bow shape in the longitudinal direction. Otherwise, a hub of a cassette half is likely deformed or ejected out, because the magnetic recording medium shrinks to excessively tighten the hub. Further, when a magnetic recording medium such as an audio-tape recorded with music is exposed to a high temperature, not only the output level greatly varies in a reproduction process to cause abnormal reproduction, but also the magnetic recording medium shows poor running property. Especially in the case of using a magnetic recording tape having a thin support (e.g., audio compact cassette tapes such as C-80 type, C-90 type and C-120 type), the above-mentioned troubles caused by the shrinkage of tapes markedly take place, because the magnetic tape encased in a cassette half is so long that the winding times of tape around the hub increases.
The audio cassette tape is used for recording music in recent years. In this case, a magnetic recording medium is required to show satisfactory frequency characteristics and a superb reproducibility of original sounds. Therefore, the occurrence of the above troubles becomes serious problem in the practical use.
In the case of a video cassette tape, very high density-recording is now possible by shortening the recording wavelength or narrowing a track width. A magnetic recording medium is, therefore, required to show excellent electromagnetic conversion characteristics such as high video outputs, high A/N ratio and excellent reproducibility of originally recorded pictures. Further, the recent wide use of portable VTRs request a tape to have an improved running property and durability. While tapes need to be highly durable, the tape thickness tends to be thinner, for example, less than 20 .mu.m.
Therefore, both of an audio-tape and a video-tape are strongly required to have much more improved electromagnetic conversion characteristics, running property and durability than the conventional ones.